Electro-optic modulators, typically lithium niobate modulators, are essential components of most lightwave systems. In state of the art commercial systems, these devices are digital and operate with data transmission rates up to 10 Gbits. Systems under development are expected to reach data rates of 40 Gbits or more. However, as bit rates increase, drive voltages increase as well. Excessive drive voltages currently limit practical implementation of ultra high bit rate modulators. New device designs, with reduced drive voltages, are critical to the development and large-scale commercial application of these modulators.
Among proposals for achieving lower drive voltages in Mach-Zehnder electro-optic modulators is the optimization of the region of interaction between the optic and electric fields. A technique that has had limited success is the etching of ridges into the lithium niobate crystal so the waveguide is located in a rdige of electro-optic material. This improves the field penetration and allows the drive voltage to be reduced. This structure also reduces the RF line capacitance in the interaction region. However, the profiles of the optical and electric fields in this device structure remain quite different, which prevents optimal interaction over the available interaction region.